Our ability to non-destructively hold and move objects within a living cell, with our optical trap, provides a opportunity to precisely measure the intra-cellular forces acting on these objects. However, measuring such forcesis not a trivial matter because of the irregular shape and composition of most biological structures. Since we are primarily concerned with chromosomes, we are using isolated and fixed chromosomes from CHO cells to calibrate our trap. First we determined the force produced by the trap on latex spheres of a know size and in a medium of known viscosity. To make this force determination, a sphere was trapped and the microscope stage moved at a constant velocity ( approximately 2f/min) while the power of the trap was reduced until the sphere escaped. By knowing the power level of the trap when sphere escaped, the viscosity of medium, and the sphere size the force could then be calculated based on Braggs laws of drag. We are currently substituting chromosomes for spheres and repeating this calibration. This is an ongoing project.